The present invention relates generally to protective devices for telecommunication equipment and more particularly to secondary protectors for high voltage and current transients that may occur on the telephone tip or ring lines.
High voltage transients may occur due to lightning strikes, power line switching or the like and can cause deleterious affects on modern telecommunication equipment including facsimile machines, modems and the like. Zener diodes, gas discharge tubes and MOV's are among the conventional devices which have been used to protect equipment such as relays and transformers from such disturbances; however, none of these are satisfactory for use with sensitive electronic equipment currently in use. Zener diodes, for example, are not suitable for transient suppression since they are designed for DC dissipation having relatively low thermal capacitance and are not designed to handle high pulsed energy. MOV's (metal-oxide varistors) do not have a sharp current initiation and thus their usefulness is limited by the amount of leakage current which can be tolerated. MOV's also degrade as a result of operation which ultimately leads to a loss of protection. Gas discharge tubes do not react quickly enough to provide effective protection to fast rising voltage wave forms.
Recently a new series of surge protectors has become available which provides improved solid state telephone line protection. Known as the TISP series of dual symmetrical transient voltage suppressors, manufactured and sold by Texas Instruments Incorporated, they comprise two bidirectional suppressor elements connected to a common terminal. They will suppress voltage transients between the ring terminal and common, the tip terminal and common and between the ring and tip terminals. The TISP protector is based on a four layer thyristor structure whose gate region contains a diffused section that acts as a zener. At low current levels the TISP protector limits the overvoltage similar to a zener diode. Higher current levels result in a crowbar action by the thyristor portion shunting excess energy away from the equipment. The device gives bidirectional protection for any wire to ground and wire to wire.
The device comprises a chip as shown in FIGS. 1 and 2 having four protector sections, two for positive voltage (A+, B+) and two for negative voltage (A-, B-) protection. All four sections have a common connection C at the bottom of the chip which normally is connected to ground.
Having a symmetrical construction the zener voltage, V.sub.z, between A and C and B and C is the same. However, the protection voltage level from A to B is two times V.sub.z.
In applications where the same energy capability is required for all combinations separate devices are required as indicated in FIG. 3; however, this arrangement is undesirable since it results in unbalanced turn-on. Further, it is desirable that the protector be provided in a single package but for the FIG. 3 arrangement this would be difficult to achieve since there is no electrically common mounting surface, that is, the heat sinks of the chips are electrically isolated. The resulting lead frame would be considerably complex as well as having limited heat sink area available for each chip.
It is therefore an object of the present invention to provide a transient voltage protector for telecommunication equipment having the same protection capabilities between the two telephone lines and between each line and ground with balanced turn-on and which is readily manufacturable and small in size.
It is another object to provide a transient voltage protector which also has current limiting capability. Conventionally, when current limiting is desired a fuse element or PTC (positive temperature coefficient of resistivity) element is separately placed in series with the equipment to be protected and the transient voltage protector.
Briefly, in accordance with a first embodiment of the invention, first and second solid state chips, each having four laterally disposed protector sections, two for positive voltage protection and two for negative voltage protection and having a bottom portion connected to all four sections is thermally and electrically coupled to a common heat sink. The A section of one is coupled to one telephone line and the B section of the other is coupled to the other telephone line with the other of the A and B sections electrically coupled together. This results in two protector sections being effective for any of the three transient possibilities, i.e., either telephone line to ground or one telephone line to the other, providing balanced turn-on.
In another embodiment the common heat sink is a portion of a dual-in-line lead frame and two electrically isolated portions are electrically connected respectively to first and second PTC elements which are in turn electrically and thermally connected to the chips. Both the chips and the PTC elements are bonded to the lead frame using electrically and thermally conductive materials and the top portion of the PTC elements and the chips are electrically connected through wire connections. The entire assembly is encapsulated in thermally conductive and electrically insulative low stress overmold material.